fix hardcoded reference to /mnt/test in common.attr

fix hardcoded reference to /mnt/test in common.attr

When trying to run test 70, we encountered several problems.
Sergey has strace'd some of the problems to attempts to
set xattr on the wrong test dir (/mnt/test/), which appears to be
hardcoded in common.attr.

Reported-by: Sergey Ivanov <sergey57@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>